I'm assuming that your doing a rectangle skirt? How long you like your skirts might depend your personal preferences and how tall/short you are. I'm around 5'2"/3" and I wear my basic skirts around 19 to 20 inches long.
If your talking about how wide your fabric should be so you'll to be able to fit a petticoat under it I normally do three times my natural waist measurement. So my waist measurement is, say 27 inches so that would be 81 inches. [And FYI when my pieces are all cut out I end up with two pieces measuring around 40"x20" give or take]
How much fabric you'll need has to do with your measurements, what way the direction the print is going on your chosen fabric, and the width of the fabric. I mostly get fabrics that are 45" wide so when it comes to a non-directional printed fabric for a simple no frills/ruffle skirt I would get no less than 2 yards when shopping because either way I cut it (width wise or long wise) its going to be enough. Of course if your bigger and/or you wanna do ruffles and make matching accessories you might want to get more fabric obviously!
Hope I read your question correctly. :3